Aluminum alloy-boron carbide composite material

ABSTRACT

A cast composite material is prepared by furnishing an aluminum-based matrix alloy and forming a mixture of free-flowing boron carbide particles and the aluminum-based matrix alloy in molten form which is stirred to wet the matrix alloy to the boron carbide particles and to distribute the particles throughout the volume of the melt. The molten mixture is then cast. The fluidity of the molten mixture is maintained by (a) maintaining the magnesium content of the matrix metal below about 0.2% by weight, or (b) starting with a matrix metal containing less than 0.2% by weight magnesium and adding further magnesium to the mixture a short time before casting, or (c) having at least 0.2% by weight titanium present in the mixture.

TECHNICAL FIELD

This invention relates to cast metal matrix composite materials and,more particularly, to a cast aluminum alloy-boron carbide metal matrixfor use as a structural material, particularly as a neutron absorbingmaterial.

BACKGROUND ART

There is a great interest in the nuclear energy industry forconstruction materials which will absorb, and therefore not release,neutrons, e.g. in containers for waste fuel. Boron carbide has been usedfor many years in the nuclear industry as a neutron absorbing materialand is a commercially available material meeting ASTM standards. Boroncarbide reinforced metal matrix composites also have application aslightweight structural materials.

International Application WO 00/26921 filed in the name of ReynoldsMetals Company and published on May 11, 2000, describes the use of analuminum alloy-boron carbide composite as a neutron absorbent materialfor storage of both hot section and spent nuclear fuel. These compositeproducts were prepared by a powder metallurgical technique in whichaluminum alloy powder was first mixed with boron carbide particles. Thepreferred aluminum alloy for the matrix was found to be an AA6000 seriesalloy, such as AA6010 alloy, which was mixed with at least 15% boroncarbide particles. The AA6000 series alloys contain at least 0.25% Mgand

AA6010 contains at least 0.8% Mg. The reference found Al, Mg and Si tobe acceptable elements, while finding AA2000, AA3000 and AA7000 alloysto be undesirable.

Powder metallurgy is an expensive technique for manufacturing largeindustrial components as required for the nuclear industry. There is,therefore, a need for a simpler and less expensive method for producingaluminum alloy-boron carbide composite products. Skibo et al. U.S. Pat.No. 4,786,467 describes a method of making aluminum alloy composites inwhich a variety of non-metallic particles are added to the aluminumalloy matrix. Among a wide variety of non-metallic particles that werementioned, boron carbide was included. However, no tests are shown usingboron carbide and the tests were conducted primarily with siliconcarbide particles. In the Skibo method, the silicon carbide particleswere mixed into a molten aluminum alloy and the mixture was then stirredto wet the aluminum alloy to the particles. The mixed material was thencast.

It was found that there can be problems of reaction between certainfiller particles and the metal alloy matrix, such as is described inHammond et al. U.S. Pat. No. 5,186,234. That patent was overcoming aproblem encountered in certain situations where the molten compositematerial cast very poorly, had low fluidity and resulted in anunacceptable product. This was particularly a problem in a foundryremelter for holding molten composites containing SiC in an aluminummatrix.

It was found that certain alloying elements can inhibit wetting of therefractory particles in a metal matrix composite, such as is describedin Skibo U.S. Pat. No. 5,083,602, in which case such wettabilityinhibiting elements were added after the alloy had wetted the particles.This did not address the problem of attack on the refractory bymagnesium during wetting, since magnesium was described as useful inencouraging wetting in the first (wetting) step.

Lloyd et al. EP 0 608 299 describes a procedure where aluminum particlesare dispersed in an aluminum alloy containing about 0.15 to 3% Mg wherestrontium is added to suppress the formation of spinal phase, whichotherwise forms and depletes the matrix of available magnesium.

Hansson et al. U.S. Pat. No. 5,246,057 describes a procedure wherealumina particles are dispersed in an aluminum alloy containing aninitially high Mg concentration to produce a stable spinel coating onthe alumina which is subsequently reduced to the desired magnesium levelby dilution.

Ferrando et al. U.S. Pat. No. 5,858,460 describes a method of producinga cast composite for aerospace applications using boron carbide in amagnesium-lithium or aluminum-lithium alloy wherein a silver metalliccoating is formed on the particle surfaces before mixing them into themolten alloy. This was done to overcome a problem of poor wettability ofthe particles by the alloy and reactivity.

Pyzik et al. U.S. Pat. No. 5,521,016 describes a method of. producing analuminum-boron carbide composite by infiltrating a boron-carbide preformwith a molten aluminum alloy. The boron carbide is initially passivatedby a heat treatment process.

Rich et al. U.S. Pat. No. 3,356,618 describes a composite for nuclearcontrol rods formed from boron carbide or zirconium diboride in variousmetals where the boron carbide is protected by a silicon carbide ortitanium carbide coating applied, for example by chemical vapourdeposition, before forming the composite. The matrix metals are hightemperature metals however, and do not include aluminum alloys.

Jason S. H. Lo, CA 2,357,323 describes a composite for brakeapplications formed from a preform of refractory particles, whiskers orfibres which is infiltrated (e.g. by squeeze casting) with an aluminumalloy containing 1 to 40% binary intermetallic particles formed byadding a second metal powder to the aluminum alloy before infiltration.The intermetallic particles are formed both in the molten aluminum andalso in heat treatments of the finished composite. The refractoryparticles include boron carbide and the second metal includes titanium.

DISCLOSURE OF THE INVENTION

Attempts were made by the present inventors to make aluminum alloy-boroncarbide composite products in accordance with Skibo et al. U.S. Pat. No.4,786,467. However, only very limited amounts of boron carbide particlescould be added to the molten aluminum before the mixture became tooviscous to be cast. It has been found according to the present inventionthat the problem is the presence of magnesium in the metal matrix. Thus,it has been found that an aluminum alloy-boron carbide composite forstructural, e.g. neutron absorption, applications can retain itsfluidity by (a) maintaining the magnesium content of the matrix metalbelow about 0.2% by weight or (b) starting with a matrix metalcontaining less than 0.2% by weight magnesium and adding furthermagnesium to the mixture a short time before casting or (c) having atleast 0.2% by weight titanium present in the mixture. The composite inits broadest aspect may contain from about 10 to about 40 volume percentof free-flowing boron carbide particles and from about 90 to about 60volume percent of molten aluminum alloy.

When fluidity is controlled by maintaining the magnesium content below0.2% by weight, the magnesium content is preferably less than about 0.1%by weight and more preferably less than about 0.05% by weight.

During holding of the molten composite, reactions are believed to occurwhich lower the fluidity of the composite. Both wrought alloys andfoundry alloys can be used if the low magnesium criteria is applied.

Thus, the present invention in one aspect provides a method ofmanufacturing a cast composite material, comprising the steps of:providing an aluminum-based matrix alloy containing less than about 0.2weight percent magnesium; preparing a mixture of from about 10 to about25 volume percent of free-flowing boron carbide particles and from about90 to about 75 volume percent of the molten matrix alloy; stirring themolten mixture to wet the aluminum alloy to the boron carbide particlesand to distribute the particles throughout the volume of the melt; andcasting the molten mixture.

The aluminum composite casting obtained is well adapted for such furtheroperations as (a) remelting and casting a shape, (b) extrusion and (c)rolling or (d) forging.

For producing wrought aluminum alloys, a preferred composition is analloy of the AA1000 series having less than 0.2 weight percentmagnesium. For foundry alloys a preferred composition is an aluminumalloy containing about 5 to 10% by weight silicon and less than 0.2% byweight magnesium.

The amount of boron carbide added is typically the highest amountpossible that will permit castability. This is generally in the range of10 to 25% by volume in the composite and preferably about 15 to 20% byvolume.

Even under conditions of relatively low magnesium in the compositionthere is a tendency for the aluminum alloy matrix to react with theboron carbide over time, and therefore limit the usefulness of thecomposite since delays in casting and excessive holding times on remeltcan inevitably occur. There is also a limit to the amount of boroncarbide that can be added under such conditions and this limit is lessthan what can normally be used in other less reactive situations.Finally the limitation on magnesium levels limits the scope ofapplications somewhat since magnesium imparts certain desirablemechanical properties to metal matrix composites.

The composites and method of production can therefore be furthermodified to permit longer holding times (either in the initialproduction or in remelting and casting operations—thus making themparticularly preferred for remelting and casting operations such as mayoccur during foundry casting of parts or in recycling of scrapmaterials) and/or higher boron carbide loadings. These modifications tothe invention include (a) having at least 0.2 weight percent titaniumpresent in the mixture and/or (b) adding further magnesium to themixture a short time before casting.

Thus the present invention in a further aspect provides a method ofmanufacturing a composite material, comprising the steps of: providingan aluminum-based matrix alloy containing at least 0.2 weight percenttitanium; preparing a mixture of from about 10 to about 40 volumepercent of boron carbide particles and from about 90to about 60 volumepercent of molten matrix alloy; stirring the molten mixture to wet thealuminum alloy to the boron carbide particles and to distribute theparticles throughout the volume of the melt; and casting the moltenmixture.

This aspect of the invention also provides a cast composite productcomprising an aluminum alloy containing a uniform distribution of boroncarbide particles dispersed in the aluminum alloy matrix where theconcentration of boron carbide particles is from 10 to 40 volume percentand the concentration of titanium in the composite is at least 0.2weight percent of the aluminum plus titanium.

For obtaining a composite with titanium in the above amounts, it isconvenient to use an AA1xxx alloy containing titanium.

In yet a further aspect of the invention there is provided a method ofmanufacturing a composite material, comprising the steps of: providingan aluminum-based matrix alloy containing less than 0.2 weight percentmagnesium; preparing a mixture of from about 10 to about 25 volumepercent boron carbide particles and from about 90 to about 75 volumepercent of molten matrix alloy; stirring the molten mixture to wet thealuminum alloy to the boron carbide particles and to distribute theparticles throughout the volume of the melt; adding magnesium to themolten mixture; and casting the molten mixture within 20 minutes ofadding the magnesium; wherein the amount of added magnesium raises themagnesium concentration in the aluminum alloy matrix to between 0.2 and0.8 weight percent.

This aspect of the invention also provides a cast composite materialcomprising an aluminum alloy containing between 0.2 and 0.8 weightpercent magnesium and between 10 and 25 volume percent boron carbiderefractory particles dispersed in the alloy matrix.

According to a still further aspect of the invention there is provided amethod of manufacturing a composite material, comprising the steps of:providing an aluminum-based matrix alloy containing less than 0.2 weightpercent magnesium and at least 0.2 weight percent titanium; preparing amixture of from about 10 to about 40 volume percent of boron carbideparticles and from about 90 to about 60 volume percent of molten matrixalloy; stirring the molten mixture to wet the aluminum alloy to theboron carbide particles and to distribute the particles throughout thevolume of the melt; adding magnesium to the molten mixture; and castingthe molten mixture within 30 minutes of adding the magnesium; whereinthe amount of added magnesium raises the magnesium concentration in thealuminum alloy matrix to between 0.2 or more weight percent.

This aspect of the invention also provides a cast composite materialcomprising an aluminum alloy containing 0.2 or more weight percentmagnesium and between 10 and 40 volume percent boron carbide refractoryparticles dispersed in the alloy matrix, and at least 0.2 weight percenttitanium in the composite based on the total aluminum plus titanium. Thealuminum alloy matrix is preferably substantially free of aluminideintermetallics. The term “dispersed” means that the particles aredistributed substantially uniformly throughout the matrix, typical of aparticles distributed by stirring.

The aluminum alloy referred to hereinbefore is preferably selected fromwrought alloys such as AA2xxx, AA3xxx, AA4xxx or AA6xxx, or castingalloys such as AA2xx or AA3xx plus the added titanium.

It is also particularly preferred that the magnesium be present in anamount of no more than 1.4 weight percent of the matrix alloy.

In the preceding embodiments containing added magnesium, it is preferredthat the magnesium be added after the molten mixture has been stirred sothat the wetting of the aluminum alloy to boron carbide is complete andthe particles are distributed through the melt and it is particularlypreferred that the magnesium be added while the composite is beingtransferred from the mixing vessel to the casting machine. For example,this may be done in a casting trough or in a transfer ladle. It is alsopreferred that the composite be mixed during the period of time betweenadding the magnesium and casting the product. This mixing is preferablycarried out both in the vessel in which the composite is formed and inthe trough or transfer ladle used to convey the composite to a castingmachine for casting a product.

In the preceding embodiments it is particularly preferred that the boroncarbide refractory be added as a free-flowing powder to the moltenaluminum and that the mixing be carried out in a manner that limits theamount of gas entrained in the composite.

The preceding embodiments are useful as structural materialsparticularly for neutron absorbing applications. A minimum boron carbidecontent of 10 volume percent is needed to provide useful neutronabsorbing properties. The upper level of boron carbide is dictated bythe fluidity requirements of the mixture and it is preferable to limitthis to 25 volume percent in situations where the titanium is not addedto improve fluidity or, in cases of titanium additions, where themagnesium level is above 0.2% by weight.

The mixture is amenable to any form of casting including DC casting ofbillets or slabs, casting or ingots for future remelting and casting, orcasting into shapes using any convenient form of shape casting.

In the composites containing added titanium, the titanium is preferablypresent in part as an intermetallic compound coating at least part ofthe surfaces of the boron carbide refractory particles. Theintermetallic compounds may additionally contain either boron or boronplus aluminum. The refractory particles are present as a uniformdispersion of particles typical of powders that are free-flowing powdersadded to an alloy mixture with stirring.

Although not wishing to be bound by any theory, it is believed that theaddition of titanium causes a reaction with the surface of the boroncarbide particles to form a stable titanium-containing compound on thesurface that does not disperse in the matrix and prevents further attackby the aluminum alloy in the matrix. These compounds contain boronand/or carbon in addition to titanium and may have a variety ofstoichiometric or non-stoichiometric compositions. Thus the compositecan be held for extended periods of time without loss of fluidity causedby the gradual formation of aluminum carbides etc., and at the sametime, higher concentrations of boron carbide can be added without lossof fluidity before casting. The stabilized boron carbide is also moreresistant to attack by magnesium containing alloys.

It has been found that at titanium levels of less than 0.2 weightpercent measured with respect to the total aluminum plus titanium, thestabilizing effect is insufficient to overcome a gradual loss offluidity. It is believed that this may relate to inadequate coverage ofthe particle surfaces by the stabilizing layer of titanium containingmaterials.

It is believed that the surface stabilizing titanium-containingmaterials are more stable, for example, than Al—Ti intermetalliccompounds. At the titanium levels used in the present invention,relatively little titanium is in solution and, absent the boron carbide,the remaining titanium would be present as an Al—Ti intermetallic.However, in the present invention such Al—Ti intermetallics appear to beconverted to a large extent to the surface stabilizing compounds and fewif any Al—Ti intermetallic particles can be found in the metal matrix.Higher levels of titanium therefore increase the stabilizing effect andthe useful upper limit to the titanium concentration is that which isneeded to coat and stabilize the boron carbide particles. Beyond thatlevel, additional titanium is expected to form of titanium aluminidesthat may eventually produce unacceptable material properties.Accordingly, the maximum titanium level used in this invention ispreferably no more than 5 weight percent based on the total aluminumplus titanium content.

Although the titanium effectively stabilizes the particles, magnesium inthe alloy can displace one or more of the titanium surface compounds andstart to degrade the particles. Therefore, the composites containing 0.2weight percent or more of magnesium must be held for a limited period oftime before casting, and it is preferred that alloys containing moderateamounts of magnesium be used (AA2xxx, AA3xxx, AA4xxx, AA6xxx, AA2xx orAA3xx) and most preferably that the amount of magnesium be limited to amaximum of 1.4 weight percent.

It will be understood that the titanium concentrations given in theforegoing description, whether with reference to the matrix alloy or thetotal composite, represent titanium in all forms. It is known that thereis a definite solubility limit of titanium in aluminum and above thatlimit excess titanium comes out of solution as intermetallics orrefractory compounds, including titanium-boron compounds. Thus an alloyor composite that is specified as containing at least 0.2% titaniumincludes the titanium in solution plus titanium in the form of Ti—Al orTi—Al—B or Ti—B containing compounds. In both the alloy and compositethe percentage titanium is determined on the basis of the total weightof titanium present divided by the weight of all aluminum alloyingcomponents including the total titanium. The titanium can be added inany convenient form, including master alloy (for example an Al-10% Timaster alloy) or as titanium containing granules or powders.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph showing a composite according to the presentinvention containing added titanium in an aluminum (AA1xxx) matrix;

FIG. 2 is a titanium element map of a portion of the micrograph of FIG.1;

FIG. 3 is a micrograph of a composite according to the present inventionhaving an aluminum (AA6xxx) alloy matrix without added titanium; and

FIG. 4 is a micrograph of a composite according to the present inventionhaving the same matrix as FIG. 3 but with added titanium.

BEST MODES FOR CARRYING OUT THE INVENTION EXAMPLE 1

Using the technique of U.S. Pat. No. 4,786,467, composites were preparedin which boron carbide particulate was mixed with matrix alloys havingthe following compositions: TABLE 1 Matrix Alloy Si Fe Cu Mg Zn Ti Alloy1 (wt %) 0.54 0.23 0.23 0.91 — 0.004 Alloy 2 (wt %) 0.04 0.63 0.0850.004 0.006 0.015

In the case of Alloy 1 (containing 0.91% Mg), upon addition of only 7.5%by volume of boron carbide, the mixture became highly viscous and couldnot be stirred further.

In the case of Alloy 2 (containing only trace amounts of Mg), 15% byvolume of boron carbide was added without any stirring problems. Thiscomposite material was then held for one hour and was still sufficientlyfluid for casting. Samples taken from the cast ingots using Alloy 2 wererolled on a laboratory rolling mill to thickness of between 1.65 and2.20 mm, and the samples were subjected to neutron absorption tests. Anaverage neutron attenuation coefficient of 1.06 μm/mm was obtained.

EXAMPLE 2

A further composite was prepared according to the methods outlined inU.S. Pat. No. 4,786,467 in which 15% by volume of boron carbideparticulate was combined with an Al-8.7% Si alloy containing only 0.01%Mg by weight.

This material was held for 1.5 hours and still retained sufficientfluidity to be cast easily. The matrix alloy was similar to alloysdisclosed in U.S. 5,186,234 commonly used for manufacture of metalmatrix composites using SiC reinforcement, except that the Mg level wasagain at trace level rather than the normal level of at least 0.3% byweight specified for such alloys.

EXAMPLE 3

A further set of composites was prepared according the methods in U.S.Pat. No. 4,786,467 in which 15% by volume of boron carbide particulatewas added to AA1xxx and AA4xxx base alloy containing various addedconcentrations of titanium. The fluidity of the resulting mixture afterholding the composite for various times was tested by casting thecomposite into a book mould in the form of an elongated horizontal strip6 mm thick having spaced restrictions every 36 mm along the strip, wherethe restriction reduced the thickness to 3 mm at that point. Thedistance that the composite flowed along the mould before solidifyingwas a measure of its fluidity. A “fluidity” of greater than 50 mm wasconsidered acceptable.

In Table 2 fluidity measurements at various holding times are given fora base aluminum alloy containing 0.02 wt % Si, 0.13wt % Fe, 0.003 wt %Cu, 0.002 wt % Mg, 0.001 wt % Mn, 0.002 wt % Zn. The base alloy alsocontained 0.001 wt % Ti and to this alloy varying amounts of titaniumwere added up to 2.0 wt % Ti. The results in Table 2 show that for lessthan 0.2 wt % Ti, the fluidity falls in time and becomes unacceptableafter holding for about one hour. At 0.2 wt % Ti, the fluidity remainsuseable for up to one hour and for increasing titanium additions, thestability of fluidity time increases rapidly. A value of 240 mmrepresents complete filling of the mould. TABLE 2 (Fluidity in mm)Holding time (minutes) Ti 10 min 20 min 40 min 60 min 0.001 wt % 120 mm 90 mm  65 mm  30 mm 0.2 wt % 130 mm  95 mm  70 mm  50 mm 0.5 wt % 212mm 155 mm  97 mm  70 mm 1.0 wt % 240 mm 240 mm 205 mm 195 mm 1.5 wt %240 mm 240 mm 240 mm 240 mm 2.0 wt % 240 mm 240 mm 240 mm 240 mm

In Table 3 fluidity measurements at various holding times are given fora base aluminum alloy of the AA4xxx type containing 4.2 wt % Si, 0.12wt% Fe, 0.06 wt % Cu, 0.02 wt % Mg, 0.16 wt % Mn, 0.003 wt % Zn. The basealloy also contained 0.07 wt % Ti and to this alloy varying amounts oftitanium was also added to make 1 wt % Ti in the matrix. Again thefluidity of the mixture at low titanium became unusable at 60 minutes,whereas 1 wt % Ti gave a high degree of stability. TABLE 3 (Fluidity inmm) Holding time (minutes) Ti 10 min 20 min 40 min 60 min 0.07 wt % 120mm  90 mm  60 mm  35 mm 1 wt % 240 mm 240 mm 240 mm 205 mm

A sample of the composite in Table 2 having 1 wt % titanium and takenafter 10minutes holding was examined metallographically and the resultsare shown in FIGS. 1 and 2. The composite shows the presence of boroncarbide particles decorated with small precipitated particles coveringthe surfaces. An elemental map (FIG. 2) shows that this layer containstitanium. More detailed analysis of the layer showed that it was formedof Ti and B or C containing compounds (nominally TiB₂ or TiC). Thespaces between the particles showed no Ti-Al intermetallics which wouldnormally be present in abundance in a 1 wt % Ti containing aluminummatrix.

EXAMPLE 4

A matrix having the AA6351 composition except for magnesium was preparedand 15 volume% boron carbide powder mixed into it to create a composite.An amount of magnesium sufficient to provide 0.6 wt % Mg in the matrixwas immediately added into the mixing vessel after mixing the boroncarbide powder and the liquid composite was held and stirred with animpeller at 500 rpm. The torque applied to the impeller was measured intime. Table 4 shows the torque (in arbitrary units) that was developedin time. TABLE 4 Holding time after Mg addition (min) 0 5 10 20 40Torque developed 32 37 46 50 57 (arbitrary units)

From experience, a torque of greater than 50 units at 500 rpm means thatthe castability of the mixture has deteriorated excessively. The aboveexample shows that at 0.6% added Mg this occurs after about 20 minutesafter the magnesium addition. However, if the composite is cast withinabout 20 minutes of the magnesium addition, it retains sufficientfluidity for casting.

EXAMPLE 5

A composite based on an AA6xxx matrix and containing 15 vol % boroncarbide particles was prepared with different levels of titanium andwhere magnesium was added after specified holding times. A base aluminumalloy containing 1.0 wt % Si, 0.11 wt % Fe, 0.001 wt % Cu, 0.002 wt %Mg, 0.01 wt % Zn was prepared. The base alloy also had 0.001 wt % Ti.The fluidity measured as in Example 3 at 20 and 40 minutes holding timesafter preparation of the initial composite with low Ti and with 1 wt %titanium at which time magnesium sufficient to give 0.8 wt % Mg in thematrix was added, and the fluidity again measured after 2 to 5 minutesfurther mixing. The 2 to 5 minutes is typical of the residence time in ametallurgical casting trough. Table 5 shows that the addition of 0.8 wt% Mg to an alloy with low titanium causes excessive deterioration of thefluidity in a short time, whereas 1% of titanium stabilizes thecomposite sufficiently than even 0.8 wt % Mg additions can be cast morereadily that composites low in both Ti and Mg. TABLE 5 (Fluidity in mm)Titanium Magnesium Holding time (Minutes) addition addition 20 min 40min 0.001 wt % None  90 mm  65 mm 0.001 wt % 0.8 wt %  60 mm  35 mm 1 wt% None 240 mm 205 mm 1 wt % 0.8 wt % 160 mm 110 mm

Samples were examined metallographically after 40 minutes holding timein both cases. FIG. 3 represents the composite where no Ti was addedprior to addition of boron carbide and FIG. 4 represents the compositewhere 1% Ti was added prior to addition of boron carbide (in accordancewith present invention). In FIG. 3 there is substantial attack on theboron carbide and reacted aluminum carbide crystals are evident in thecomposite. In FIG. 4, the protective titanium containing layer ispresent on many of the particles and the attack on the boron carbide ismuch less and localized.

1. A method of preparing a cast composite material, comprising the stepsof: providing an aluminum-based matrix alloy; preparing a molten mixtureof from about 10 to about 40 volume percent of free-flowing boroncarbide particles and from about 90% to about 60 volume percent of amelt of said aluminum-based matrix alloy; stirring the molten mixture towet the matrix alloy to the boron carbide particles and to distributethe particles throughout the volume of the melt; and casting the moltenmixture to form a cast composite material; characterized by maintainingthe fluidity of the molten mixture by limiting the Mg in thealuminum-based matrix alloy to below 0.2% by weight, at least untilcompletion of said distribution of said particles throughout said volumeof the melt.
 2. A method according to claim 1, characterized in that themixture is prepared with an amount of said boron carbide within therange 10 to 25 volume percent.
 3. A method according to claim 1,characterized in that the mixture is prepared with an amount of saidboron carbide within the range 15 to 20 volume percent.
 4. A methodaccording to claim 1, characterized by limiting said magnesium contentto less than 0.1% by weight.
 5. A method according to claim 1,characterized by limiting said magnesium content to less than 0.05% byweight.
 6. A method according to claim 1, characterized in that the meltis prepared with a matrix alloy that is either an AA1000 type alloy oran aluminum alloy comprising 5 to 10% by weight Si.
 7. A methodaccording to claim 1, characterized in that further magnesium is addedto the molten mixture a short time before casting.
 8. A method accordingto claim 7, characterized in that the amount of said further magnesiumadded to the molten mixture raises the magnesium concentration in thealuminum matrix alloy to between 0.2% and 0.8% by weight.
 9. A methodaccording to claim 7, characterized in that the further magnesium isadded in a casting trough or in a transfer ladle.
 10. A method accordingto claim 7, characterized in that the mixture is further stirred afterthe addition of the magnesium.
 11. A method according to claim 7,characterized in that the aluminum alloy employed to prepare the mixtureis selected from the group consisting of AA2xxx, AA3xxx, AA4xxx, AA6xxx,AA2xx or AA3xx alloys.
 12. A method according to claim 7, wherein moltenmixture is cast within 20 minutes of adding the further magnesium.
 13. Amethod according to claim 1, characterized in that the molten mixturecontains Ti in an amount of at least 0.2% by weight but no more than 5%by weight.
 14. A method according to claim 1, characterized in thatfurther magnesium is added to the molten mixture to raise the magnesiumconcentration in the aluminum matrix alloy to between 0.2% and 1.4% byweight.
 15. A method according to claim 13, characterized in that thealuminum alloy is selected from the group consisting of AA2xxx, AA3xxx,AA4xxx, AA6xxx, AA2xx or AA3xx alloys with added titanium.
 16. A methodaccording to claim 13, characterized in that the aluminum matrix alloyis an AA1xxx alloy with added titanium.
 17. A method according to claim14, characterized in that the molten mixture is cast within 30 minutesof adding the further magnesium.
 18. A method of preparing a castcomposite material, comprising the steps of: providing an aluminum-basedmatrix alloy; preparing a molten mixture of from about 10 to about 40volume percent of free-flowing boron carbide particles and from about90% to about 60 volume percent of a melt of said aluminum-based matrixalloy; stirring the molten mixture to wet the matrix alloy to the boroncarbide particles and to distribute the particles throughout the volumeof the melt; and casting the molten mixture to form a cast compositematerial; characterized by maintaining the fluidity of the moltenmixture by providing at least 0.2% by weight but no more than 5% byweight of Ti in the aluminum-based matrix alloy.
 19. A method accordingto claim 1, characterized in that the cast mixture is remelted and castinto a shape.
 20. A method according to claim 1, characterized that thecast mixture is extruded into a shape.
 21. A method according to claim1, characterized that the cast mixture is rolled.
 22. A method accordingto claim 1, characterized that the cast mixture is forged.
 23. A methodaccording to claim 1, characterized that the cast mixture is formed intoa neutron absorbing material.
 24. A cast composite product comprising analuminum alloy matrix having between 10 and 40 volume percent of boroncarbide refractory particles dispersed therein, said compositecontaining at least 0.2 weight percent but no more than 5 weight percenttitanium, said titanium forming a stable titanium-containing compound onthe surface of the boron carbide particles that is not dispersed in thematrix and prevents attack by the aluminum alloy in the matrix, and thealuminum alloy matrix having an as-cast microstructure.
 25. A castcomposite product according to claim 24, wherein the aluminum alloymatrix is an A1xxx alloy.
 26. A cast composite product according toclaim 24, wherein the aluminum alloy matrix contains at least 0.2 weightpercent magnesium.
 27. A cast composite product according to claim 24,wherein the aluminum alloy matrix is an AA2xxx, AA3xxx, AA4xxx, AA6xxx,AA2xx or AA3xx alloy.
 28. A cast composite product comprising analuminum alloy matrix having between 10 and 25 volume percent of boroncarbide refractory particles dispersed therein, said compositecontaining at least 0.2 weight percent magnesium, said compositeexhibiting substantially no aluminum carbide crystals at the surfaces ofthe refractory particles when examined metallographically, and thealuminum alloy matrix having an as cast microstructure.
 29. A castcomposite according to claim 28, wherein the aluminum alloy is anAA2xxx, AA3xxx, AA4xxx, AA6xxx, AA2xx or AA3xx alloy.
 30. A castcomposite according to claim 28, wherein the composite contains no morethan 0.8 weight percent magnesium.
 31. A cast composite productcomprising an aluminum alloy matrix having between 10 and 40 volumepercent of boron carbide refractory particles dispersed therein, whereinthe aluminum alloy matrix contains at least 0.2 weight percentmagnesium, the composite contains at least 0.2 weight percent titaniumbut no more than 5 weight percent, said titanium forming a stabletitanium-containing compound on the surface of the boron carbideparticles that is not dispersed in the matrix and prevents attack by thealuminum alloy in the matrix, and the aluminum alloy matrix has an ascast microstructure.
 32. A cast composite according to claim 31, whereinthe aluminum alloy is an AA2xxx, AA3xxx, AA4xxx, AA6xxx, AA2xx or AA3xxalloy.
 33. A cast composite according to claim 31, wherein the aluminumalloy matrix contains no more than 1.4 weight percent magnesium.